March 31st, 2011
BURIED SECRETSPreserve Infrastructure, by Catherine Higgins.
A UDOT Research Division study team recently unearthed some galvanized steel from retaining walls to see how the important reinforcement material is holding up over time.
Mechanically stabilized earth walls or MSE walls are commonly built around bridges or used as retaining walls along freeways. UDOT has a database of over 700 walls in use across the state-owned transportation system. MSE walls are expected to have a useful life of 75 to 100 years — a long time to be in place as a critically important roadway feature.
MSE walls are built in “lifts” or layers of fill with steel or geosynthetic reinforcement installed between layers — so it’s not often that engineers can actually see how the reinforcement holds up over time.
The walls are “a valuable asset,” says Chief Geotechnical Engineer Keith Brown. “We need to maintain walls the best way we can.”
I-15 was reconstructed in Salt Lake County prior to the 2002 Olympics, and UDOT used that reconstruction as an opportunity; construction contractors were required to insert steel wire “coupons” that could eventually be removed and examined. All coupons were made of galvanized steel – the same material used to fabricate the wire mesh reinforcement used in MSE walls.
Galvanization, which applies zinc to steel, does not improve steel strength. Zinc corrodes more slowly than steel and therefore delays the eventual corrosion of steel. The byproduct of zinc corrosion also slows the corrosion of steel.
Engineers require steel of a large enough diameter to assure that proper reinforcement is in-tact at the end of MSE wall life, after some expected corrosion takes place.
Field observations and lab tests
Extractable steel wire coupons provided a hands-on view of the condition of MSE reinforcement materials. “This is an actual physical inspection,” says BYU Assistant Professor Travis Gerber, who led the study. Other methods use equipment that measures the condition of coupons in place. “It’s more accurate to actually see and measure” the removable coupons.
Field observations showed the galvanization on coupons to be intact with a variable amount of white oxidation visible on the surface. Laboratory measurements showed that the average thickness of galvanization on all coupons presently exceeds standards required at the time of installation.
Gerber’s team, with UDOT oversight, was successful at establishing a critical baseline index of the condition of steel reinforcements used in MSE walls along I-15 through Salt Lake County. Because the study involved actual physical inspection, data from the study is accurate and reliable.
In addition, Gerber believes that study conclusions can apply to other nearby locations. “We think you can extrapolate the results,” as long as soils, backfill specifications and environmental area are similar, “the performance should be similar.”
The conclusions from this study will eventually contribute to better design choices, appropriate material use, and effective long term maintenance standards and practices.